The present invention relates to a process for producing acyloxysilanes, and more particularly, to an improved process for the continuous production of acyloxysilanes in a system which substantially increases capacity over prior art systems and forms recyclable hydrogen chloride.
Acyloxysilanes are well known cross-linking agents for one-part room temperature vulcanizable silicone rubber compositions. One common acyloxysilane cross-linking agent is methyltriacetoxysilane.
Acyloxysilane cross-linking agent has been made by the reaction of an appropriate chlorosilane with a carboxylic anhydride or with a carboxylic acid. One process for preparing acyloxysilanes by reacting a chlorosilane with a carboxylic acid or carboxylic acid anhydride in the presence of an iron complexing agent is disclosed in U.S. Pat. No. 3,974,198. In U.S. Pat. No. 3,974,198, an aliphatic carboxylic acid, such as glacial acetic acid, was added at the top of a distilling column to a refluxing mixture of chlorosilane in an organic solvent, such as benzene, containing an iron complexing agent. After the completion of the addition of the aliphatic carboxylic acid, the solvent was removed by distillation, and the acyloxysilane compound was eventually isolated from the mixture.
Aliphatic carboxylic acid in the vapor phase is passed upwards from the bottom of a column filled with Raschig rings countercurrent to a flow of chlorosilane in U.S. Pat. No. 4,176,130. However, in U.S. Pat. No. 4,176,130 only a limited amount of aliphatic carboxylic acid is introduced into the column so that the feed rate of the carboxylic acid does not exceed 1.3 moles per gram atom of silicon-bonded chlorine in the column. Thus, at most, only a very slight excess of aliphatic carboxylic acid is introduced into the column in U.S. Pat. No. 4,176,130. The liquid glacial acetic acid vaporizes as it enters the column, and the acetic acid vapor rises against the flow of the chlorosilane. When only a slight excess or less of the aliphatic carboxylic acid is used in the reaction, it is believed that much of the chlorosilane remains unreacted, and the column throughput is low. Furthermore, the passage of the aliphatic carboxylic acid vapor upwards only from the bottom of the column does not insure a sufficient amount of the carboxylic acid in the upper regions of the column where the chlorosilane is introduced into the column, to react with chlorosilane vapors in the upper regions of the column. Consequently, significant amounts of chlorosilane vapor can be lost or remain unreacted at the top of the column.
The formation of dimer at the bottom of the column is a significant side reaction that results from thermal decomposition of monomeric acyloxysilanes or from a reaction of the chlorosilane with acyloxysilane or both. The dimer is represented by the following formula: EQU R.sub.n (R'COO).sub.3-n Si-O-Si(R'COO).sub.3-n R
wherein R and R' are alkyl radicals generally of 1 to about 8 carbon atoms and n is 1 to 3. When the reactants form dimer, it reduces the amount of acyloxysilane product. Thus, it is desirable to reduce the amount of dimer formed from the reaction, or to increase column throughput to provide commercial quantities of the acyloxysilanes even though the dimer is formed. It is also desirable to provide sufficient aliphatic carboxylic acid in the upper regions of the column to react with chlorosilane vapors before they escape or condense.
The continuous prior art processes are also disadvantageous insofar as low boiling contaminants generally accumulate in the column, and the low boiling contaminants reduce the column temperature. Accordingly, it is desirable to provide a process which eliminates this problem so that the column temperature can be maintained at a steady maximum temperature. In prior art processes, especially those using reduced pressure, where hydrogen chloride gas is produced as a by-product, it is necessary to neutralize the gas and thereby waste a valuable by-product. Accordingly, it is desirable to provide a process which recovers hydrogen chloride gas suitable for recycling and use in other processes.